1. Field of the Invention
The present invention relates to LED fabrication technology and more particularly, to a flip-chip GaN-based LED fabrication method, which enables a high reliable GaN LED chip to be bonded to a thermal substrate having a large heat dissipation area with a conducting adhesive, facilitating heat dissipation, enhancing the luminous efficiency of the GaN LED chip and prolonging its lifespan.
2. Description of the Related Art
Many different types of light emitting apparatus are commercially available. Following the trend of the next generation for green energy, LED (light emitting diode), more particularly, white LED has been intensively used in street lamps, tunnel lamps, hand lamps, sign boards, home lightings and backlight modules for LCD panel for the advantages of power-saving, small size, high stability and high reliability characteristics.
Commercial white light emitting devices commonly use blue GaN-based LED chips to match with yellow, green or red phosphor. However, a sapphire substrate has the drawback of poor heat transfer characteristic. Temperature will affect he reliability and lifespan of sapphire substrate-based LED chips. According to the luminous efficiency of regular LED devices, about 50%˜60% of the power will be turned into heat. Further, according to conventional LED module fabrication methods, phosphor is mixed with epoxy resin or silicon adhesive subject to a predetermined ration and then the mixture thus prepared is coated on LED chips. During operation of a conventional LED device, waste heat will be accumulated in the inside of the LED device, lowering the luminous efficiency and shortening the lifespan. An overheat may result in device burnout. Therefore, the factor of heat dissipation efficiency has been greatly emphasized. If waste heat cannot be quickly dissipated during operation of a LED package product, the life cycle and reliability of the product will be badly affected. To facilitate quick dissipation of waste heat, heat pipe, heat-transfer block, heat sink and/or radiation fin may be used with a LED module. However, the use of these attached devices relatively increases the product size, reducing the advantages of small size and light weight of LED products.
FIG. 12 illustrate a GaN LED chip according to the prior art, which comprises a support frame A8, a sapphire substrate A1 supported on the support frame A8 and fixedly bonded thereto with an adhesive, an N-type GaN ohmic contact layer A2 formed on the sapphire substrate A1, a light-emitting layer A3 formed on the N-type GaN ohmic contact layer A2, a P-type GaN ohmic contact layer A4 formed on the light-emitting layer A3, a translucent conducting layer A5 formed on the P-type GaN ohmic contact layer A4 for distributing electric current and enhancing luminous efficiency, a P-type electrode pad A6 and an N-type electrode pad A7 respectively formed on the translucent conducting layer AS and the N-type GaN ohmic contact layer A2, and gold or aluminum wires A9 electrically connecting the P-type electrode pad A6 and an N-type electrode pad A7 to external contacts A10. According to this design, a part of the light emitted by the light-emitting layer A3 goes through the P-type electrode pad A6 outside the translucent conducting layer A5 to the phosphor in the packaged shell. Because the P-type electrode pad A6 blocks a part of the light emitted by the light-emitting layer A3, the luminous efficiency of the LED package is lowered.
To eliminate the aforesaid electrode pad light-blocking problem, U.S. Pat. No. 5,557,115 discloses an improved LED design, entitled “Light emitting semiconductor device with sub-mount”, as shown in FIG. 13, which employs flip chip technology to increase the effective light-emitting area. According to this design, the light emitting semiconductor device comprises a sapphire substrate B1, a buffer layer B2 and an N-type GaN ohmic contact layer B3 formed in proper order on the sapphire substrate B1, a light-emitting layer B4 and a P-type GaN ohmic contact layer B5 formed in proper order on the center area of the N-type GaN ohmic contact layer B3, a P-type electrode pad B6 connecting the P-type GaN ohmic contact layer B5 to an external heat-transfer substrate, N-type electrodes B7 disposed at two opposite lateral sides relative to the light-emitting layer B4, and an N-type electrode pad B8 connecting one N-type electrode B7 to the external heat-transfer substrate. Thus, the light-emitting surface B11 is fully opened for emitting light efficiently. However, the use of the metallic P-type electrode pad B6 and N-type electrode pad B8 to reflect light tends to cause an increase of the forward voltage, lowering the luminous efficiency.
FIG. 14 illustrates still another prior art LED design disclosed in U.S. Pat. No. 6,514,782, entitled “Method of making an III-nitride light-emitting device with increased light generating capability”. According to this design, LED dies (chips) C1 are electrically connected to solder contacts C11; C21 of a circuit board C2 by means of heat-transfer blocks C3, such as gold balls or gold-tin solder bumps. This method has the drawback of high manufacturing cost. As the electrode pads in the flip-chip LED device are electrically connected to the circuit board and capable of reflecting the light emitted by the light-emitting layer toward the sapphire substrate, the metallic property of the electrode pads tends to cause an increase in the forward voltage, thereby lowering the luminous efficiency. This prior art design discloses the formation of a metallic reflection layer on the circuit board, however, the far distance between the metallic reflection layer and the light-emitting layer causes an attenuation of the emitted light.
FIG. 15 illustrates still another prior art LED design disclosed in U.S. Pat. No. 6,514,782, which comprises a sapphire substrate D1, an N-type GaN ohmic contact layer D2, a light-emitting layer D3, a P-type GaN ohmic contact layer D4, a translucent conducting layer D5 and conducting metallic reflection layer D6. Further, the N-type GaN ohmic contact layer D2 and the translucent conducting layer D5 are respectively and electrically connected to the conducting metallic reflection layer D6 and a circuit board D8 by electrodes D7. Further, in order to prevent conduction between the electrodes D7 and the conducting metallic reflection layer D6 and to protect the conducting metallic reflection layer D6 against current leakage, a polyimide insulation layer D9 is filled in the gaps. However, it is difficult to form the polyimide insulation layer D9 without affecting the relative surface elevation between the electrodes D7 and the conducting metallic reflection layer D6. A significant elevational difference between the electrodes D7 and the conducting metallic reflection layer D6 will lead to a connection error between the LED and the circuit board, lowering the yield rate.
FIG. 16 illustrates a flip-chip GaN-based LED design according to Taiwan Patent M350824. According to this design, an etching technique is employed to divide an epitaxial layer into a first epitaxial layer portion El and a second epitaxial layer portion E2 so that the P-type electrode pad E3 and the N-type electrode pad E4 can be approximately maintained at the same elevation. However, in order to reduce the contact impedance between the N-type electrode pad E4 and the second epitaxial layer portion E2, the N-type electrode pad E4 must be extended to form an ohmic contact with an N-type GaN ohmic contact layer E21. This designs enables the he P-type electrode pad E3 and the N-type electrode pad E4 to be approximately maintained at the same elevation, facilitating circuit board connection. However, the metallic reflection layer E5 round the conducting layer E6 tends to increase the forward voltage. Further, this design does not teach any measure to protect the surface of the edge of the grooves in the LED chip.
Further, the aforesaid prior art designs cannot eliminate accumulation of waste heat to lower the luminous efficiency due to high consumption of power. The use of polyimide insulation layer to minimize power consumption relatively lowers heat dissipation performance. Further, if the P-type electrode pad and the N-type electrode pad are not maintained at the same elevation, circuit board connection may fail, lowering the yield rate.
Therefore, it is desirable to provide a flip-chip LED, which eliminates the drawbacks of the aforesaid prior art designs.